Extend an iov array to cover an additional VIRTIO descriptor, up to a maximum length
#include <qvm/vio.h>
ssize_t vio_append_iov_cap(const vdev_t *vdp, vq_t *vq, unsigned didx, struct vio_iov *iov, size_t cap)
If successful, the length of the new descriptor; otherwise, a negative errno value.